The monograph examines the church reform of Patriarch Nikon, the consequence of which was the church schism. The main attention is paid to the correction of the liturgical text carried out under Patriarch Nikon: the textual picture of the correction of two liturgical books, Trebnik and Chasoslov, the textual features of the Donikon and Nikon texts, as well as the role and place of correction of the liturgical text in a number of reasons that led to the church schism.